One of the things climate change or global warming deniers like to do is to use unadjusted temperature data to try and ‘prove’ that temperatures aren’t increasing and that the 1930s were just as warm as now. I posted the following to a Facebook page (Australian politician Craig Kelly’s) in response to a few denier comments about homogenised data just being part of the climate change ‘fraud’.

No use looking at raw data. It has to be adjusted or homogenised data. This is done by weather bureaus all around the world to take out non-climatic changes such as shifting weather stations and changing measuring methods. Three main things have to be adjusted for.

Over the decades, weather stations have typically moved from post offices in the centre of town, where it is warmer especially at night (urban heat island effect) to airports on the outskirts of town in green open fields where it’s cooler. This is why raw temperatures in the 1930s are often about as high as now (the move to airports was generally from about the 1940s onwards). This non-climatic change to temperatures has to be taken out to get a true picture of how temperatures have changed.

It can go the other way too; for example, town-based weather stations that haven’t moved might now be surrounded by more concrete and steel than decades past.

Also, temperature measuring methods have changed and this can have an effect. In the 19th century, instruments were typically attached to walls of buildings and protected from the sun by metal screens but this had the effect of pushing up temperatures. Instruments were then put in gardens away from buildings but infrared radiation from the ground could push up temps on calm sunny days. High-tech automatic weather stations that counter these sort of problems are now common.

Once all the adjustments are made, we can get a far more accurate picture of how temperatures have actually changed rather than changes influenced by the shifting of weather stations or different measuring methods.